A Home Office pathologist, whose evidence wrongly helped to convict solicitor Sally Clark for the murder of her two baby sons, will be accused of 'failing in his duty' as an expert witness.

At a hearing tomorrow before a General Medical Council (GMC) professional conduct committee, Dr Alan Williams, a forensic pathologist, who has regularly given evidence in crown court cases, is to answer claims that he failed to make available all of the medical reports about a post-mortem on one of Mrs Clark's babies.

He is accused of changing his opinion about the post-mortem, failing to give an adequate cause of death and not discharging the duties of a 'competent pathologist'.

The GMC in Manchester will also hear a claim that 'Williams carried out examinations that he was not competent to perform and he failed to provide comprehensive expert evidence in advance for a trial'.

He was criticised following Mrs Clark's successful appeal against her conviction last year. Defence lawyers and doctors at her trial were not made aware that one of her babies, eight-week-old Harry, who died in 1998, had suffered a staphylococcus aureus infection in eight parts of his body, which could have been the cause of death.

Clark's 11-week-old son, Christopher, died in 1996. It was shortly after Harry's death that Clark was arrested, as experts had assumed Christopher had died of natural causes.

The judge at Clark's appeal said Williams' decision not to pass on all the test results had carried a 'significant risk of a miscarriage of justice'.

Lord Justice Kay said the appeal court did not believe Williams was 'knowingly being a party to the putting forward of a false case', but branded his explanation for not revealing all the test results 'wholly unacceptable'.

Penny Mellor, who campaigned for Clark's release, said the hearing raised questions about expert witnesses. 'This case clearly demonstrates the urgent need for an overhaul of how experts are appointed and what qualifications they must have, particularly in cases where a defendant faces imprisonment or the permanent loss of a child.'